Magpul ten round mag modified to hold five???

[keith13b]

I assume that using p-mags, you just swap the follower and adjust the feed lips as needed? Why not just get a 5-rnd limiter for a 10 round p-mag and adjust the limiter to only hold 5 Grendel rounds? Pardon my ignorance, but it sounds fairly simple.

I have some AR-Stoner 5-round 6.5G mags. Never used them, but they dry cycle fine. I know you wanted polymer; just tossing that out there.